The use of integrating cavities, or xe2x80x9cintegrating spheres,xe2x80x9d to diffuse electromagnetic radiation is known in the prior art. These cavities are for example used in spectrophotometers and other optical instrumentation to diffuse source intensity nonuniformities and various polarization states. Integrating cavities are sometimes used to approximate a Lambertian radiator or as an integrator of incident radiant flux.
As the optical sensitivities of modem detectors and optical fibers increase, the qualifications needed in integrating cavities also increase. By way of example, when the output of a polarization dependent optical component (e.g., an optical fiber) is measured by a detector through an integrating cavity, it is desirable to minimize any influence the cavity may have on measured polarization dependencies.
A further complication of the prior art is that optical cavities are often used by manually coupling one component, e.g., an optical fiber, to the cavity. The resulting signal through the cavity is quite dependent on the physical placement of the component with the cavity, making it quite difficult to achieve repeated test measurements.
There is therefore the need to provide an improved integrating cavity for optical measurement. The invention overcomes the problems in the prior art by providing, in one feature, an integrating optical cavity system that reduces polarization and input emission variations. Other features of the invention will be apparent in the description that follows.
In one aspect, the invention provides an integrating optical system for measuring optical radiation. The system has a first sphere (forming a xe2x80x9cprimaryxe2x80x9d integrating cavity) and a second sphere (forming a xe2x80x9csecondaryxe2x80x9d integrating cavity). An optical fiber interfaces to an input aperture of the first sphere so that light from the fiber enters the first sphere. A detector interfaces with the second sphere such that light from the first sphere couples to the detector by scattering within the first and second spheres and without a direct line of sight between the detector and the input aperture. The secondary integrating cavity has a smaller volume than the primary integrating cavity.
In the preferred aspect of the invention, each of the inner walls of the spheres is made from SPECTRALON(copyright) material from Labsphere. The secondary integrating cavity is made smaller so as to decrease losses incurred by light scattering transmission through the first and second spheres. The detector is preferably configured so that it does not receive xe2x80x9cspecularxe2x80x9d radiation (i.e., radiation from a single reflection) from the walls of the primary cavity. Those skilled in the art should appreciate that the first and second integrating cavities may take different shapes other than spheres without departing from the scope of the invention.
In one aspect, the system utilizes an off center entrance cone as an input port for the optical fiber. The detector, likewise, may also be off-center as a matter of design choice.
The invention provides certain advantages. First, the detector is physically decoupled from the first sphere and is therefore not in direct line of sight to the input aperture. This configuration reduces effects of polarization orientation and of input patterns due to rotation about the longitudinal dimension of the input optical fiber; accordingly, the configuration provides for enhanced repeatability in production test environments. More particularly, the lack of polarization response allows measurement of polarization dependent loss in fiber optic components by varying the input state of polarization. A detector in the form of a low polarization response meter provides for monitoring change in output associated with the input polarization of the component under test,, and not for the test equipment polarization sensitivities. Moreover, in the preferred aspect, the invention permits full capture of diverging radiation from the optical fiber. Finally, the invention provides for high repeatability in test measurements.
In another aspect, the invention provides for wavelength detection. The system of this aspect includes a colored filter in front of the detector. A detector measurement is made to generate detector current output. Other filters and detectors may be used in sequence (or concurrently, as described below) to generate a ratio of detector currents with wavelength.
In still another aspect, the invention provides a test system that has broad and flat spectral response characteristics, e.g., utilizing InGaAs and Si detectors.
In yet another aspect, a second detector couples with the second sphere so as to provide further reduction in polarization response. By way of example, the responses from the two detectors may be averaged together; the detectors may also be mounted ninety degrees from one another. Additional detectors may be coupled with the second sphere, and in different orientations, in accord with the invention.
In one aspect, the first sphere has a cone shaped cut-out formed with the input aperture, to accommodate the numerical aperture diverging light of the optical fiber.